They did it because more and more of them saw the essential wrongness of it In the USA, while there was an awful civil war over it, keep in mind that the war, as much of it as was driven by slavery, was because the slave-holding minority in the south feared it being outlawed by the non-slave majority in the north.

The numbers amazed me - I certainly had not thought that the percentage of enslaved human beings basically exceeded one half of Virginia's population (when you exclude the essentially non-slave owning counties which became West Virginia).